The present invention relates to information processing techniques, and more particularly to techniques that enable two or more parties to discover mutually held information without disclosing the private or confidential information of the parties.
There are several situations where two parties need to exchange information with one another to determine information that is held in common by both parties. As part of the information exchange, the parties typically also desire that information that is not held in common by the parties (i.e., information that is held by one party but not the other party) be not disclosed to the other party.
One example of such a situation occurs in the email marketing business. The use of electronic mail messages (“emails”) as marketing tools has gained widespread popularity in recent years. Emails provide a convenient channel for marketers to reach a large audience of potential consumers. Marketers generate and maintain lists of email addresses and send emails to addresses in the list. The emails may comprise information marketing/advertising vendors' products, hyperlinks (e.g. URLs) to the vendors' websites, and the like. Marketers also commonly run email campaigns targeted against email address lists of another marketer (referred to as the “rental affiliate” or “affiliate”). In such a scenario, the affiliate sends emails on behalf of the marketer to email addresses included in the address list maintained by the affiliate. 
The recent enactment of laws aimed to reduce spam is requiring marketers to change the manner in which email campaigns are conducted, especially campaigns targeted against address lists of a rental affiliate. For example, under the recently enacted federal anti-SPAM law (“CAN-SPAM”), emails marketers are required to suppress (i.e., not send emails to) unsubscribed email addresses from all email campaigns they initiate. This includes email campaigns implemented by and targeted against the list of a list rental affiliate.
Accordingly, when a marketer runs an email campaign against a list of email addresses of an affiliate, the new laws require that emails not be sent to email addresses that occur both in the marketer's list of addresses to be suppressed and in the affiliate's list of email addresses. Accordingly, in order to abide by the new laws, it is essential to determine email addresses that occur in the marketer's suppression list and the affiliate's list. These email addresses that are in both lists represent information that is held in common by the two parties, namely the marketer and the affiliate. However, in the process of determining these common addresses, neither the affiliate nor the marketer wants their non-common information (e.g., addresses in the affiliate's list that are not in the marketer's suppression list and email addresses in the marketer's suppression list that are not in the affiliate's list) to be disclosed to the other party.
Historically, affiliates have offered suppression services for an additional fee. Typically, a marketer would send a list of email addresses to be suppressed (the “suppression list”) to an affiliate and the affiliate would remove any email addresses found in the suppression list from the affiliate's list prior to conducting the email campaign. However, this approach violates the privacy policies of many marketers, which prohibit them from sharing customer data with third parties such as the affiliate.
According to another technique, the affiliate sends its list of email addresses to the marketer. The marketer then removes email addresses from the affiliate list that are to be suppressed and returns the reduced list of addresses back to the affiliate. The affiliate then conducts email campaigns against only those email addresses in the reduced list received from the marketer. While this approach does not expose the marketer's customer data to the affiliate, it exposes the affiliate's customer data to the marketer. This is generally unacceptable to the affiliate and may violate its privacy policies. 
Accordingly, a challenge in implementing suppression of email addresses between an affiliate and a marketer is that both parties view their data as private. Neither the affiliate nor the marketer wishes to disclose their customer and suppression information to the other party, and yet both have to cooperate to meet the legislative requirements of suppressing unsubscribed addresses.